1. Field of the Invention
This invention relates to the field of electrostatography and, more particularly, to improvements in apparatus for controlling toner replenishment.
2. Description of Prior Art
In electrostatography, electrostatic images formed on a dielectric recording element are rendered visible via the application of pigmented, thermoplastic particles known as toner. Typically, such toner forms part of a two-component developer mix consisting of the toner particles and magnetically-attractable carrier particles to which the toner particles adhere via triboelectric forces. During the development process, the electrostatic forces associated with the latent image act to strip the toner particles from their associated carrier particles, and the partially denuded carrier particles are returned to a reservoir.
Several techniques are known for replenishing the spent development mix with fresh toner. They include monitoring the toner concentration in the developer mix, monitoring the amount of toner applied to the recording member during development, and monitoring the number of character print signals applied to a print head.
Whatever the replenishment method, when toner is added to the developer mix, the added toner participates in the development process only after some delay determined by the flow and mixing patterns of the toning station, the tribo-charging characteristics of the toner, and any time periods during which the station is idle. Such a delayed response can be experienced in the time between addition of toner and its being sensed by a concentration monitor. A delayed response can also be experienced in the time between addition of toner and some response in the toned image. In addition, the effects of removal of toner by the imaging process manifest themselves after a similar delay. During the duration of such a delay, any estimate of the amount of toner in the toning station will be in error, since the change in the amount of toner in the station is, in effect, not detectable to the sensing means.
There are two traditional methods known in the prior art for handling this problem. First, the controlling mechanism can be forced to wait between replenishment cycles for a period (referred to herein as "dead time") sufficient to insure that the toner in the station is well mixed and charged and is detectable by the sensing means. If this waiting period is much longer than the image frame period, many frames are toned before fresh toner can be added. Indeed, the waiting period may be measured in image frames. This permits substantial amounts of toner take-out and large individual additions of fresh toner. The result is an undesirable amount of toner concentration variation.
A second method known in the prior art for controlling replenishment measures the toner concentration more frequently and does not wait for the toner in the station to be well mixed and charged; but the method artificially limits the amount of toner added in order to minimize periodic over-concentrations. This detuning technique, in effect, results in the use of a lower-than-optimum control system gain and slows the response of the control system.